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<div class="chapter">
<div class="titlepage"><div><div><h1 class="title">
<a name="concepts"></a>Chapter 1. Concepts</h1></div></div></div>
<div class="toc">
<p><b>Table of Contents</b></p>
<dl class="toc">
<dt><span class="section"><a href="concepts.html#concepts.callpolicies">CallPolicies</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="concepts.html#concepts.callpolicies.introduction">Introduction</a></span></dt>
<dt><span class="section"><a href="concepts.html#concepts.callpolicies.callpolicies_composition">CallPolicies
        Composition</a></span></dt>
<dt><span class="section"><a href="concepts.html#concepts.callpolicies.concept_requirements">Concept
        Requirements</a></span></dt>
</dl></dd>
<dt><span class="section"><a href="concepts/dereferenceable.html">Dereferenceable</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="concepts/dereferenceable.html#concepts.dereferenceable.introduction">Introduction</a></span></dt>
<dt><span class="section"><a href="concepts/dereferenceable.html#concepts.dereferenceable.concept_requirements">Concept
        Requirements</a></span></dt>
</dl></dd>
<dt><span class="section"><a href="concepts/extractor.html">Extractor</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="concepts/extractor.html#concepts.extractor.introduction">Introduction</a></span></dt>
<dt><span class="section"><a href="concepts/extractor.html#concepts.extractor.concept_requirements">Concept Requirements</a></span></dt>
<dt><span class="section"><a href="concepts/extractor.html#concepts.extractor.notes">Notes</a></span></dt>
</dl></dd>
<dt><span class="section"><a href="concepts/holdergenerator.html">HolderGenerator</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="concepts/holdergenerator.html#concepts.holdergenerator.introduction">Introduction</a></span></dt>
<dt><span class="section"><a href="concepts/holdergenerator.html#concepts.holdergenerator.concept_requirements">Concept
        Requirements</a></span></dt>
</dl></dd>
<dt><span class="section"><a href="concepts/resultconverter.html">ResultConverter</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="concepts/resultconverter.html#concepts.resultconverter.introduction">Introduction</a></span></dt>
<dt><span class="section"><a href="concepts/resultconverter.html#concepts.resultconverter.resultconverter_concept_requirem">ResultConverter
        Concept Requirements</a></span></dt>
<dt><span class="section"><a href="concepts/resultconverter.html#concepts.resultconverter.resultconvertergenerator_concept">ResultConverterGenerator
        Concept Requirements</a></span></dt>
</dl></dd>
<dt><span class="section"><a href="concepts/objectwrapper.html">ObjectWrapper</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="concepts/objectwrapper.html#concepts.objectwrapper.introduction">Introduction</a></span></dt>
<dt><span class="section"><a href="concepts/objectwrapper.html#concepts.objectwrapper.objectwrapper_concept_requiremen">ObjectWrapper
        Concept Requirements</a></span></dt>
<dt><span class="section"><a href="concepts/objectwrapper.html#concepts.objectwrapper.typewrapper_concept_requirements">TypeWrapper
        Concept Requirements</a></span></dt>
<dt><span class="section"><a href="concepts/objectwrapper.html#concepts.objectwrapper.caveat">Caveat</a></span></dt>
</dl></dd>
</dl>
</div>
<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="concepts.callpolicies"></a><a class="link" href="concepts.html#concepts.callpolicies" title="CallPolicies">CallPolicies</a>
</h2></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="concepts.html#concepts.callpolicies.introduction">Introduction</a></span></dt>
<dt><span class="section"><a href="concepts.html#concepts.callpolicies.callpolicies_composition">CallPolicies
        Composition</a></span></dt>
<dt><span class="section"><a href="concepts.html#concepts.callpolicies.concept_requirements">Concept
        Requirements</a></span></dt>
</dl></div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="concepts.callpolicies.introduction"></a><a class="link" href="concepts.html#concepts.callpolicies.introduction" title="Introduction">Introduction</a>
</h3></div></div></div>
<p>
          Models of the CallPolicies concept are used to specialize the behavior
          of Python callable objects generated by Boost.Python to wrapped C++ objects
          like function and member function pointers, providing three behaviors:
        </p>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem">
              <code class="computeroutput"><span class="identifier">precall</span></code> - Python argument
              tuple management before the wrapped object is invoked
            </li>
<li class="listitem">
              <code class="computeroutput"><span class="identifier">result_converter</span></code> -
              C++ return value handling
            </li>
<li class="listitem">
              <code class="computeroutput"><span class="identifier">postcall</span></code> - Python argument
              tuple and result management after the wrapped object is invoked
            </li>
<li class="listitem">
              <code class="computeroutput"><span class="identifier">extract_return_type</span></code>
              - metafunction for extracting the return type from a given signature
              type sequence
            </li>
</ol></div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="concepts.callpolicies.callpolicies_composition"></a><a class="link" href="concepts.html#concepts.callpolicies.callpolicies_composition" title="CallPolicies Composition">CallPolicies
        Composition</a>
</h3></div></div></div>
<p>
          In order to allow the use of multiple models of CallPolicies in the same
          callable object, Boost.Python's CallPolicies class templates provide a
          chaining interface which allows them to be recursively composed. This interface
          takes the form of an optional template parameter, <code class="computeroutput"><span class="identifier">Base</span></code>,
          which defaults to <code class="computeroutput"><span class="identifier">default_call_policies</span></code>.
          By convention, the <code class="computeroutput"><span class="identifier">precall</span></code>
          function of the <code class="computeroutput"><span class="identifier">Base</span></code> is
          invoked after the <code class="computeroutput"><span class="identifier">precall</span></code>
          function supplied by the <code class="computeroutput"><span class="identifier">outer</span></code>
          template, and the <code class="computeroutput"><span class="identifier">postcall</span></code>
          function of the <code class="computeroutput"><span class="identifier">Base</span></code> is
          invoked before the <code class="computeroutput"><span class="identifier">postcall</span></code>
          function of the <code class="computeroutput"><span class="identifier">outer</span></code> template.
          If a <code class="computeroutput"><span class="identifier">result_converter</span></code> is
          supplied by the <code class="computeroutput"><span class="identifier">outer</span></code> template,
          it replaces any <code class="computeroutput"><span class="identifier">result_converter</span></code>
          supplied by the <code class="computeroutput"><span class="identifier">Base</span></code>. For
          an example, see <code class="computeroutput"><span class="identifier">return_internal_reference</span></code>.
        </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="concepts.callpolicies.concept_requirements"></a><a class="link" href="concepts.html#concepts.callpolicies.concept_requirements" title="Concept Requirements">Concept
        Requirements</a>
</h3></div></div></div>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
<col>
</colgroup>
<thead><tr>
<th>
                  <p>
                    Expression
                  </p>
                </th>
<th>
                  <p>
                    Type
                  </p>
                </th>
<th>
                  <p>
                    Result/Semantics
                  </p>
                </th>
</tr></thead>
<tbody>
<tr>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">x</span><span class="special">.</span><span class="identifier">precall</span><span class="special">(</span><span class="identifier">a</span><span class="special">)</span></code>
                  </p>
                </td>
<td>
                  <p>
                    convertible to <code class="computeroutput"><span class="keyword">bool</span></code>
                  </p>
                </td>
<td>
                  <p>
                    returns <code class="computeroutput"><span class="keyword">false</span></code> and
                    <code class="computeroutput"><span class="identifier">PyErr_Occurred</span><span class="special">()</span> <span class="special">!=</span>
                    <span class="number">0</span></code> upon failure, <code class="computeroutput"><span class="keyword">true</span></code> otherwise.
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">P</span><span class="special">::</span><span class="identifier">result_converter</span></code>
                  </p>
                </td>
<td>
                  <p>
                    A model of <code class="computeroutput"><span class="identifier">ResultConverterGenerator</span></code>.
                  </p>
                </td>
<td>
                  <p>
                    An MPL unary Metafunction Class used produce the "preliminary"
                    result object.
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">x</span><span class="special">.</span><span class="identifier">postcall</span><span class="special">(</span><span class="identifier">a</span><span class="special">,</span>
                    <span class="identifier">r</span><span class="special">)</span></code>
                  </p>
                </td>
<td>
                  <p>
                    convertible to <code class="computeroutput"><span class="identifier">PyObject</span><span class="special">*</span></code>
                  </p>
                </td>
<td>
                  <p>
                    <code class="computeroutput"><span class="number">0</span></code> and <code class="computeroutput"><span class="identifier">PyErr_Occurred</span><span class="special">()</span>
                    <span class="special">!=</span> <span class="number">0</span></code>
                    upon failure. Must "conserve references" even in the
                    event of an exception. In other words, if <code class="computeroutput"><span class="identifier">r</span></code>
                    is not returned, its reference count must be decremented; if
                    another existing object is returned, its reference count must
                    be incremented.
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">P</span><span class="special">::</span><span class="identifier">extract_return_type</span></code>
                  </p>
                </td>
<td>
                  <p>
                    A model of Metafunction.
                  </p>
                </td>
<td>
                  <p>
                    An MPL unary Metafunction used extract the return type from a
                    given signature. By default it is derived from <code class="computeroutput"><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">front</span></code>.
                  </p>
                </td>
</tr>
</tbody>
</table></div>
</div>
</div>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright © 2002-2005, 2015 David Abrahams, Stefan Seefeld<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>
      </p>
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